Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. A typical fuel cell includes an ion conductive electrolyte layer sandwiched between an anode layer and a cathode layer. There are several different types of fuel cells known in the art, one of which is known as a solid oxide fuel cell or SOFC.
In a typical SOFC, air is passed over the surface of the cathode layer and a fuel containing hydrogen is passed over the surface of the anode layer opposite that of the cathode layer. Oxygen ions from the air migrate from the cathode layer through the dense electrolyte layer in which it reacts with the hydrogen and CO in the fuel, forming water and CO2 and thereby creating an electrical potential between the anode layer and the cathode layer of about 1 volt.
Each individual SOFC is mounted within a metal frame, referred to in the art as a retainer, to form a cell-retainer frame assembly. The individual cell-retainer frame assembly is then joined to a metal separator plate, also known in the art as an interconnector plate, to form a fuel cell cassette. In order to produce a voltage sufficiently high to be used in variety of applications, the cassettes are stacked in series to form a fuel cell stack.
Along one side of each fuel cell cassette, between the SOFC and the outside edge of the retainer and separator plate, a plurality of anode supply passages are formed through the retainer and the separator plate. In the fuel cell stack, the plurality of anode supply passages for each fuel cell cassette together form a plurality of anode supply chimneys which allow fuel supplied at one end of the stack to be communicated to other end of the stack, thereby distributing fuel to each SOFC. The plurality of anode supply passages may be formed at regular intervals along the length of the fuel cell cassette to distribute the fuel evenly across the surface of each SOFC. Along the side opposite the side of each fuel cell cassette with the anode supply passages, between the SOFC and the outside edge of the retainer and the separator plate, a plurality of anode exhaust passages are formed through the retainer and the separator plate. In the fuel cell stack, the plurality of anode exhaust passages for each fuel cell cassette together form a plurality of anode exhaust chimneys which allow anode exhaust from each fuel cell cassette to be communicated to one end of the fuel cell stack. The plurality of anode exhaust passages may be formed at regular intervals along the length of the fuel cell cassette in the same way as the anode supply passages.
A plurality of cathode supply passages are formed through the retainer and the separator plate along the side of each fuel cell cassette which includes the plurality of anode supply passages. In the fuel cell stack, the plurality of cathode supply passages for each fuel cell cassette together form a plurality of cathode supply chimneys which allow air supplied at one end of the stack to be communicated to other end of the stack, thereby distributing air to each SOFC. The plurality of cathode supply passages may be formed at regular intervals along the length of the fuel cell cassette to distribute the fuel evenly across the surface of each SOFC such that the plurality of cathode supply passages and the plurality of anode supply passages are in an alternating pattern along the length of the fuel cell cassette. A plurality of cathode exhaust passages are formed through the retainer and the separator plate along the side of each fuel cell cassette which includes the plurality of anode exhaust passages. In the fuel cell stack, the plurality of cathode exhaust passages for each fuel cell cassette together form a plurality of cathode exhaust chimneys which allow cathode exhaust from each fuel cell cassette to be communicated to one end of the fuel cell stack. The plurality of cathode exhaust passages may be formed at regular intervals along the length of the fuel cell cassette in the same way as the cathode supply passages.
When a large number of fuel cell cassettes are stacked, it is often difficult to balance the flow of air and the flow of fuel to each cassette which results in non-uniform mass flow of air and mass flow of fuel to each cassette. This may at least partly result from the air or fuel mass flow rate and momentum being extremely high where air or fuel enters the plurality of cathode and anode supply chimneys. Air and fuel mass gets reduced as each fuel cell cassette in the fuel cell stack draws a certain mass of air and fuel. Accordingly, air/fuel mass, velocity, momentum, and kinetic energy get reduced at the end of the stack opposite the end of the fuel cell stack that receives air from an air source. Air with high kinetic energy gets less driving force to supply air to the fuel cell cassettes closer to the air source than to the fuel cell cassettes further away from the air source. Accordingly, the fuel cell cassettes closer to the air supply receive less air than the fuel cell cassettes further away from the air source, thereby providing non-uniform flow distribution between the fuel cell cassettes. This problem is more pronounced with an increase in the number of fuel cell cassettes.
One way to provide more uniform flow distribution in fuel cell stacks is shown in U.S. Pat. No. 6,416,899. In this example, a wedge is placed in the inlet and exhaust chimneys. The wedge in the inlet chimney is oriented such that the end of the inlet chimney distal from the inlet is reduced in area. The wedge in the exhaust chimney is oriented such that the end of the outlet chimney distal from the outlet is reduced in area. While this may be effective for providing a more uniform flow distribution in the fuel cell stack, the wedges decrease the chimney size and contribute to a pressure drop in the chimney. Furthermore, there are dimensional constraints on the wedge geometry and placement accuracy and variation which limit the effectiveness of this design.
Another way to provide more uniform flow distribution in fuel cell stacks is shown in United States Patent Application Publication No. US 2003/0104265. In this example, a piercing member is inserted in a passage upstream of the inlet chimneys. While this may be effective for improving the flow distribution in the fuel cells stack, the piercing member contributes to a significant pressure drop in the inlet chimneys and has limited effectiveness due to geometric constraints.
Yet another way to provide more uniform flow distribution in fuel cell stacks is shown in U.S. Pat. No. 7,531,264. In this example, first and second manifolds are provided. The first manifold supplies a gas only to the second manifold at each cassette, but does not supply gas directly to the cassettes. The second manifold supplies the gas to each of the cassettes. While this may be effective for providing a more uniform flow distribution in the fuel cell stack, flow to individual cassettes cannot be tailored and the pressure drop required to achieve uniformity is much higher than desirable.
What is needed is a fuel cell stack with more uniform flow distribution. What is also needed is such a fuel cell stack with a low pressure drop. What is also needed is such a fuel cell stack in which flow distribution to each fuel cell cassette of the fuel cell stack can be tailored.